A time-of-flight mass spectrometer (which may be hereinafter abbreviated as the “TOFMS” according to a conventional usage) is a device in which various ions derived from a sample are made to fly a specific distance, and the time of flight is measured for each ion to calculate the mass-to-charge ratio m/z of the ion based on the fact that the flight speed of each ion depends on its mass-to-charge ratios when the ions are accelerated by the same amount of energy. In the TOFMS, the flight speed of an ion depends on the amount of initial energy imparted from an electric field or the like. Therefore, the mass-resolving power of the device is affected by the spread (variation) in the initial energy of an ion packet, i.e. a cluster of ions having the same mass-to-charge ratio.
One method for improving the energy convergence of the time of flight of ions to enhance the mass-resolving power is to adopt the configuration of a reflectron in which ions are made to fly a round trip by using an ion reflector which repels ions by an electric field. In the ion reflector, an ion having a greater amount of energy (and hence a higher flight speed) among the ions having the same mass-to-charge ratio takes a path which reaches a deeper point inside the ion reflector and turns around. Accordingly, among the ions having the same mass-to-charge ratio, an ion having a greater amount of energy effectively flies a longer distance than an ion having a smaller amount of energy, whereby the discrepancy in the time of flight due to the difference in the amount of energy is compensated for. That is to say, the energy convergence of the time of flight of the ions is improved.
However, there is a limit of the energy convergence by the ion reflector. Therefore, if the variation in the initial energy is equal to or greater than a certain extent, a considerable deviation of the flight trajectory occurs, causing a decrease in the mass-resolving power. Besides, in the reflectron TOFMS, the forward-side path along which ions fly from the ion ejector (which accelerates the ions and send them into the flight space) to the ion reflector should ideally lie on the same straight line as the return-side path along which the ions returned by the ion reflector travel to the detector. However, it is practically impossible to arrange the ion ejector and the detector on the same straight line. Accordingly, the ion ejector, ion reflector, detector and other related elements are normally arranged so that the forward-side and return-side paths will be approximately symmetrical with respect to the central axis of the ion reflector, as in a device described in Patent Literature 1 or other documents. Therefore, even the same kind of ions which have the same mass-to-charge ratio follow different flight paths if their initial positions at the time of acceleration are different. This results in a variation in the time of flight and causes a decrease in the resolving power.
To address such problems, in the TOFMS described in Patent Literature 1, a shielding plate with a slit opening having a predetermined shape and size (slit diaphragm) is placed in a free flight space in front of the detector. An ion following a deviated path which will lead to a decrease in the resolving power is thereby blocked and prevented from reaching the detector. Since the spatial spread of an ion packet is largest in the area in front of the detector, placing the shielding plate in front of the detector favorably minimizes the influence of the mechanical error of the slit opening on the resolving power. However, the use of such a shielding plate for blocking ions to improve the resolving power blocks more ions than necessary, i.e. including ions which are practically unlikely to decrease the resolving power. This causes the problem that the analytical sensitivity becomes lower due to the decrease in the amount of ions reaching the detector.
Although the TOFMS described in Patent Literature 1 is an orthogonal acceleration reflectron TOFMS, the previously described problem may also occur in a reflectron TOFMS different from the orthogonal acceleration type, such as a reflectron TOFMS in which ions are collectively ejected from an ion trap capable of accumulating ions and are made to fly, since the ion trap, ion reflector, detector and other related elements in this type of device are also normally arranged so that the forward-side and return-side paths will be approximately symmetrical with respect to the central axis of the ion reflector.